Chef::Mixin::WhyRun::ResourceRequirements

ResourceRequirements

ResourceRequirements provides a framework for making assertions about the host system's state. It also provides a mechanism for making assumptions about what the system's state might have been when running in why run mode.

For example, consider a recipe that consists of a package resource and a service resource. If the service's init script is installed by the package, and Chef is running in why run mode, then the service resource would fail when attempting to run `/etc/init.d/software-name status`. In order to provide a more useful approximation of what would happen in a real chef run, we want to instead assume that the service was created but isn't running. The logic would look like this:

# Hypothetical service provider demonstrating why run assumption logic.
# This isn't the actual API, it just shows the logic.
class HypotheticalServiceProvider < Chef::Provider

  def load_current_resource
    # Make sure we have the init script available:
    if ::File.exist?("/etc/init.d/some-service"
      # If the init script exists, proceed as normal:
      status_cmd = shell_out("/etc/init.d/some-service status")
      if status_cmd.success?
        @current_resource.status(:running)
      else
        @current_resource.status(:stopped)
      end
    else
      if whyrun_mode?
        # If the init script is not available, and we're in why run mode,
        # assume that some previous action would've created it:
        log("warning: init script '/etc/init.d/some-service' is not available")
        log("warning: assuming that the init script would have been created, assuming the state of 'some-service' is 'stopped'")
        @current_resource.status(:stopped)
      else
        raise "expected init script /etc/init.d/some-service doesn't exist"
      end
    end
  end

end

In short, the code above does the following:

runs a test to determine if a requirement is met: `::File.exist?("/etc/init.d/some-service"`
raises an error if the requirement is not met, and we're not in why run mode.
if we are in why run mode, print a message explaining the situation, and run some code that makes an assumption about what the state of the system would be. In this case, we also skip the normal `load_current_resource` logic
when the requirement is met, we run the normal `load_current_resource` logic

ResourceRequirements encapsulates the above logic in a more declarative API.

Examples

Assertions and assumptions should be created through the WhyRun#assert method, which gets mixed in to providers. See that method's documentation for examples.

Public Class Methods

new(resource, run_context) click to toggle source

# File lib/chef/mixin/why_run.rb, line 248
def initialize(resource, run_context)
  @resource, @run_context = resource, run_context
  @assertions = Hash.new {|h,k| h[k] = [] }
  @blocked_actions = []
end

Public Instance Methods

action_blocked?(action) click to toggle source

Check to see if a given action is blocked by a failed assertion

Takes the action name to be verified.

# File lib/chef/mixin/why_run.rb, line 261
def action_blocked?(action) 
  @blocked_actions.include?(action)
end

assert(*actions) click to toggle source

Define a new Assertion.

Takes a list of action names for which the assertion should be made.

Examples:

A File provider that requires the parent directory to exist:

assert(:create, :create_if_missing) do |a|
  parent_dir = File.basename(@new_resource.path)
  a.assertion { ::File.directory?(parent_dir) }
  a.failure_message(Exceptions::ParentDirectoryDoesNotExist,
                    "Can't create file #{@new_resource.path}: parent directory #{parent_dir} doesn't exist")
  a.why_run("assuming parent directory #{parent_dir} would have been previously created"
end

A service provider that requires the init script to exist:

assert(:start, :restart) do |a|
  a.assertion { ::File.exist?(@new_resource.init_script) }
  a.failure_message(Exceptions::MissingInitScript,
                    "Can't check status of #{@new_resource}: init script #{@new_resource.init_script} is missing")
  a.why_run("Assuming init script would have been created and service is stopped") do
    @current_resource.status(:stopped)
  end
end

A File provider that will error out if you don't have permissions do delete the file, *even in why run mode*:

assert(:delete) do |a|
  a.assertion { ::File.writable?(@new_resource.path) }
  a.failure_message(Exceptions::InsufficientPrivileges,
                    "You don't have sufficient privileges to delete #{@new_resource.path}")
end

A Template provider that will prevent action execution but continue the run in whyrun mode if the template source is not available.

assert(:create, :create_if_missing) do |a| 
  a.assertion { File::exist?(@new_resource.source) }
  a.failure_message Chef::Exceptions::TemplateError, "Template #{@new_resource.source} could not be found exist."
  a.whyrun "Template source #{@new_resource.source} does not exist. Assuming it would have been created."
  a.block_action!
end

assert(:delete) do |a|
  a.assertion { ::File.writable?(@new_resource.path) }
  a.failure_message(Exceptions::InsufficientPrivileges,
                    "You don't have sufficient privileges to delete #{@new_resource.path}")
end

# File lib/chef/mixin/why_run.rb, line 313
def assert(*actions)
  assertion = Assertion.new
  yield assertion
  actions.each {|action| @assertions[action] << assertion }
end

events() click to toggle source

# File lib/chef/mixin/why_run.rb, line 254
def events
  @run_context.events
end

run(action) click to toggle source

Run the assertion and assumption logic.

# File lib/chef/mixin/why_run.rb, line 320
def run(action)
  @assertions[action.to_sym].each do |a| 
    a.run(action, events, @resource)
    if a.assertion_failed? and a.block_action? 
      @blocked_actions << action
      return
    end
  end
end

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